Too, we can also get a glimpse--or in literary terminology, foreshadowing--of what may happen to Charlie by looking at Algernon. Different events in the story suggest that since Algernon had the "operashun" that Charlie was to have done, that as we see Algernon's increased intelligence and higher thinking abilities, and, eventually, decline, that we should expect the same to happen to Charlie.

Charlie will slowly transform from being a mentally-challenged man to a brilliant scholar, who is far more intelligent than the average person.  We see this transformation through his diary entries that become increasily well-written and complex.  However, the operation's effects are transitory, and he will slowly revert back to his former self.